Frictional resistance reducing vessel and a method of reducing frictional resistance of a hull

ABSTRACT

The object of the present invention is to provide a frictional resistance reducing vessel and method of reducing the frictional resistance of a hull that are able to effectively conserve energy consumed during operation by reducing frictional resistance at a low level of energy consumption. In the present invention, a negative pressure region ( 51 ), which is at low pressure relative to a gaseous space, is formed in the water accompanying operation of a hull ( 30 ), and together with bubbles being guided to this negative pressure region ( 51 ) in the water from the gaseous space, the state of this negative pressure region ( 51 ) is changed based on changes in vessel velocity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a frictional resistance reducing vessel that reduces the frictional resistance of the hull, and more particularly, to improving the total energy efficiency by efficiently releasing bubbles into the water.

[0003] 2. Description of the Prior Art

[0004] A method has been proposed in the prior art for the purpose of conserving energy consumed during operation of marine vessels and so forth that consisted of reducing frictional resistance between the hull and the water by feeding a gas into the water and juxtaposition of a large number of microbubbles in the vicinity of the surface of the hull shell plate (submerged surface).

[0005] Technologies for generating microbubbles in water are proposed in Japanese Unexamined Patent Application, First Publication No. 50-83992, Japanese Unexamined Patent Application, First Publication 53-136289, Japanese Unexamined Patent Application, First Publication 60-139586, Japanese Unexamined Patent Application, First Publication 61-71290, Japanese Unexamined Utility Model Application, First Publication No. 61-39691 and Japanese Unexamined Utility Model Application, First Publication No. 61-128185.

[0006] In these technologies, air pressurized by a pump, blower or other pressurization apparatus is blown into the water from a plurality of holes or a porous plate provided in the hull.

[0007] The present applicant has proposed a technology for juxtaposition of microbubbles on the shell plate of a hull by feeding a gas (such as air) into the water from a delivery outlet provided in, for example, the vicinity of the bow, as such a technology pertaining to reducing frictional resistance. This technology attempts to cover the hull shell plate with microbubbles as a result of dispersing microbubbles along the lines of flow of water over the hull shell plate by feeding a gas from a delivery outlet. When this gas is fed into the water, a gas supply apparatus such as a blower is used as the motive power source.

[0008] However, in the case of blowing a gas into water using such an apparatus, since a new motive power is required to operate the apparatus, the amount of motive power conserved during operation as a result of reduction by microbubbles ends up being lost. At locations of comparatively large water depths, such as the bottom of a large ship, in particular, it is necessary to pressurize the gas to a high pressure corresponding to the water pressure (hydrostatic pressure) when blowing the gas into the water, thereby resulting in the consumption of a large amount of energy. In addition, in the installation of the apparatus in the hull, huge costs are incurred, such as equipment costs and installation costs. Moreover, since the composition of the apparatus is comparatively complex, the apparatus is expensive and its maintenance and inspection are not easy.

[0009] In consideration of the above circumstances, the objects of the present invention consist of the following:

[0010] (1) to effectively conserve energy consumption during operation by reducing frictional resistance with a low level of energy consumption;

[0011] (2) to effectively reduce frictional resistance by efficiently mixing bubbles into the water;

[0012] (3) to reduce hull construction costs; and,

[0013] (4) to simplify maintenance and inspection.

SUMMARY OF THE INVENTION

[0014] The present invention employs the following means to achieve the above objects.

[0015] Namely, the present invention is characterized by being a frictional resistance reducing vessel that reduces frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; provided with a negative pressure forming portion arranged protruding from the submerged surface that forms a negative pressure region in the water which is at low pressure relative to a gaseous space; an air outlet for releasing bubbles towards the negative pressure region; a flow path of which one end is open to a gaseous space and the other end is open into the water via the air outlet; and, a drive mechanism that changes at least one of the protruding state of the negative pressure forming portion from the submerged surface, the opening surface area of the air outlet, and the flow path cross-sectional surface area of the flow path.

[0016] This frictional resistance reducing vessel is also preferably provided with a control apparatus that controls the drive mechanism based on changes in vessel speed.

[0017] In addition, the method of reducing frictional resistance of a hull as claimed in the present invention is characterized as being a method of reducing the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; wherein, a negative pressure region at a low pressure relative to a gaseous space is formed in the water accompanying vessel operation, and together with a gas being led from the gaseous space to the negative pressure region in the water, the state of the negative pressure region is changed based on changes in vessel speed.

[0018] In addition, the present invention is characterized by being a frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; provided with an outer cylindrical portion of which one end is open to the atmosphere and the other end is open in the water; an inner cylindrical portion, which together with being installed in the state of being inserted into the outer cylindrical portion, one end is open to the atmosphere and the other end is open into the water; and, an asymmetrical air blowing portion provided on the other end of the inner cylindrical portion protruding from the submerged surface, the protruding state of which is in the lengthwise direction of the hull.

[0019] This frictional resistance reducing vessel is also preferably provided with a protrusion adjustment means for adjusting the protruding state of the air blowing portion.

[0020] In addition, the cross-sectional shapes of the outer cylindrical portion and inner cylindrical portion are preferably isosceles triangles.

[0021] In addition, the present invention is characterized by being a frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; provided with a negative pressure forming portion arranged protruding from the submerged surface so that cavitation occurs in the water behind it due to the relative flow of water with respect to the hull during operation; a discharge outlet provided behind the negative pressure forming portion; and, a flow path of which one end is open to a gaseous space and the other end is open into the water via said discharge outlet.

[0022] In addition, the present invention is characterized by being a frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; provided with an indentation formed so as to be recessed from the submerged surface; a negative pressure forming member, which together with being supported to as to rotate freely inside this indentation, forms a negative pressure region in the water at a low pressure relative to a gaseous space by having at least a portion protrude from the submerged surface; a flow path for guiding air from the gaseous space to the negative pressure region in the water, one end of which being open to the gaseous space, and the other end being open into said indentation; and, an angle adjusting mechanism for causing at least a portion of the negative pressure forming member to protrude in a prescribed state from the submerged surface that supports the negative pressure forming member and adjusts the angle of the negative pressure forming member.

[0023] In addition, the present invention is characterized by being a frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; provided with an indentation provided in the submerged surface; a flow path of which one end is open to the atmosphere and the other end is open to the inside of said indentation; a wing body having a wing that is arranged within said indentation; and, a positioning mechanism that supports said wing body in a prescribed direction while allowing to move freely and positions the wing at a prescribed position; said positioning mechanism positioning the wing so that the inside of the indentation is open relative to the flow of water and results in negative pressure relative to the atmosphere during reduction of frictional resistance.

[0024] This positioning mechanism preferably positions the wing so that the lower surface of the wing is at roughly the same height as the submerged surface during times other than reduction of frictional resistance.

[0025] In addition, the above switching mechanism preferably has an operational means that allows the position of the wing to be freely manipulated from the deck.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a drawing that explains the principle of the method of reducing frictional resistance as claimed in the present invention.

[0027]FIG. 2A is a side view schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0028]FIG. 2B is an enlarged cross-sectional view of the vicinity of a bubble generator schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0029]FIGS. 3A, 3B and 3C are drawings showing the cross-sectional shape of an air induction pipe.

[0030]FIGS. 4A and 4B are drawings for explaining the manner in which the protruding height of the air induction pipe from the submerged surface is changed.

[0031]FIG. 5 is a flow chart showing an example of a procedure for changing the state of the negative pressure region in the water.

[0032]FIG. 6A is a side view schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0033]FIG. 6B is an enlarged cross-sectional view of the vicinity of a bubble generator schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0034]FIGS. 7A and 7B are drawings for explaining one example of a method of reducing the frictional resistance of a hull by a frictional resistance reducing vessel as claimed in the present invention.

[0035]FIG. 8 is a perspective view showing the structure of negative pressure forming portion 23 in FIG. 6B.

[0036]FIG. 9A is a side view of the starboard side in the vicinity of the bow schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0037]FIG. 9B is an overhead view as viewed from below of the bottom of a vessel in the vicinity of the bow of the frictional resistance reducing vessel shown in FIG. 9A.

[0038]FIG. 9C is a cross-sectional view taken along line A-A in FIG. 9A.

[0039]FIG. 10A is a side view schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0040]FIG. 10B is an enlarged cross-sectional view of the vicinity of a bubble generator schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0041]FIGS. 11A and 11B are drawings for explaining an example of a method of reducing frictional resistance of a hull by a frictional resistance reducing vessel as claimed in the present invention.

[0042]FIG. 12 is a cross-sectional view taken along line B-B in FIG. 10B.

[0043]FIG. 13A is a side view schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0044]FIG. 13B is an overhead view of the vessel bottom as viewed from in the water schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0045]FIG. 14A is an enlarged cross-sectional view of the vicinity of a bubble generator schematically showing one embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0046]FIG. 14B is an overhead view of the vessel bottom as viewed from in the water showing the constitution of a wing body in a frictional resistance reducing vessel as claimed in the present invention.

[0047]FIGS. 15A and 15B are drawings for explaining an example of a method of reducing the frictional resistance of a hull by a frictional resistance reducing vessel as claimed in the present invention.

[0048]FIG. 16 is a perspective view showing the constitution of a wing body.

[0049]FIG. 17A is an overhead view of the vessel bottom as viewed from in the water showing another embodiment of a frictional resistance reducing vessel as claimed in the present invention.

[0050]FIG. 17B is a cross-sectional view taken along line C-C in FIG. 17A.

BEST MODE FOR CARRYING OUT THE INVENTION

[0051] To begin with, an explanation is provided of the theory of reducing the frictional resistance of a hull by the technology claimed in the present invention with reference to FIG. 1. Here, for the sake of convenience, the method of reducing frictional resistance of a hull of the prior art in which a gas is pressurized and blown into the water is referred to as the “positive pressure method”, while the method utilizing negative pressure as claimed in the present invention is referred to as the “negative pressure method”. In addition, the same reference symbols are used for those portions that are the same in the remaining drawings, and an explanation of those portions will be omitted.

[0052]FIG. 1 is a drawing schematically showing frictional resistance reducing vessel 10 as claimed in the present invention. Reference symbol 11 indicates a hull shell plate (submerged surface), 12 a negative pressure forming portion, 13 an air outlet, 14 a flow path and 15 the water surface (draft line).

[0053] When frictional resistance reducing vessel 10 operates at a prescribed velocity V, water flow 20 is formed relative to the hull. Moreover, this frictional resistance reducing vessel 10 forms negative pressure region 21, which is at low pressure (negative pressure, vacuum pressure) relative to a gaseous space (air), in the water during operation. Namely, negative pressure region 21 is formed in the water as a result of the flow state of the water being changed by negative pressure forming portion 12 that protrudes from hull shell plate 11. At this tine, the pressure of air outlet 13 that faces negative pressure region 21 becomes lower than the gaseous space (air), and the force of the pressure gradient acts on the fluid (sea water and air) within flow path 14. As a result, together with sea water being discharged from flow path 14, air that has flowed in from the atmosphere flows through flow path 14, and is fed into the water in the form of microbubbles 22.

[0054] When bubbles having a volume Qv are released at a location at depth D (m) from a liquid surface in a static liquid of density ρ (the density of the bubbles is assumed to be zero), the energy required for their release is represented with the following equation:

E=(P−Pa)Qv   (1)

[0055] where, Pa is the pressure of the gaseous space (atmospheric pressure) and P is the pressure at the location where the bubbles are released (=ρgh, where g is gravitational acceleration).

[0056] At this time, if the flow rate in the vicinity of air outlet 13 on the vessel bottom is taken to be V₁, then the pressure P at that location is represented with the following equation:

P=Pa−ρ(V ₁ ² −V ²)/2+ρgD   (2)

[0057] Furthermore, although flow rate V₁ changes according to the release of bubbles to a boundary layer, this change is ignored here.

[0058] As is clear from equation (1), in the case pressure P at the location where bubbles are released is low in comparison with atmospheric pressure Pa, namely when P<Pa, energy becomes negative (E<0), and additional energy is not required for moving air to the vessel bottom. Furthermore, in addition to the bubbles that move from the atmosphere to the vessel bottom via flow path 14, the bubbles in the vicinity of air outlet 13 also include bubbles generated when the pressure of negative pressure region 21 becomes low in comparison with saturated vapor pressure as a result of cavitation and separation action that occur due to negative pressure forming portion 12.

[0059] In this manner, in frictional resistance reducing vessel 10 as claimed in the present invention, by changing the flow state of water by negative pressure forming portion 12 and forming negative pressure region 12 in the water, bubbles can be generated in the water with less energy than pressurization methods.

[0060] Next, an explanation is provided of the relationship between the shapes of the constituents for forming a negative pressure region in the water (each of the shapes, etc. of negative pressure forming portion 12, air outlet 13 and flow path 14), and frictional resistance reduction effects of the hull.

[0061] Here, each parameter is defined as shown below.

[0062] h: Mean negative pressure (Pa) at negative pressure region 21

[0063] v: Mean flow rate of air inside flow path 14 (m/sec)

[0064] Q: Quantity of air fed into water (m3/sec)

[0065] d: Protruding height of negative pressure forming portion 12 from submerged surface 11 (m)

[0066] V: Vessel velocity (m/sec)

[0067] V′: Flow rate of gas-liquid two-phase flow in vicinity of air outlet 13

[0068] ΔF: Amount of reduction of frictional resistance (kgf)

[0069] ΔR: Amount of increase in resistance due to negative pressure forming portion 12 (kgf)

[0070] ΔN=ΔF−ΔR: Frictional resistance reduction effect (Net Gain (kgf))

[0071] Here, Net Gain is equivalent to the increase in vessel velocity obtained as a result of reducing frictional resistance when converted to “kgf”.

[0072] ΔNV: Net Gain when vessel velocity V is constant

[0073] ΔNd: Net Gain when height d of negative pressure forming portion 12 is constant

[0074] α: Resistance increase coefficient when vessel velocity V is constant (kgf/m)

[0075] β: Frictional resistance reduction coefficient when vessel velocity V is constant (kgf/m)

[0076] ε: Resistance increase coefficient when height d of negative pressure forming portion 12 is constant (kgf/(m/sec)²)

[0077] γ: Frictional resistance reduction coefficient relative to square of vessel velocity V when height d of negative pressure forming portion 12 is constant (kgf/(m/sec)²)

[0078] δ: Frictional resistance reduction coefficient relative to vessel velocity V when height d of negative pressure forming portion 12 is constant (kgf/(m/sec)²)

[0079] [Relationship Between Mean Negative Pressure h (m) and Air Quantity Q (m³/sec)]

[0080] The mean negative pressure h (m) in negative pressure region 21 is proportional to the square of the flow rate v (m/sec) of air inside the air induction pipe (AIP) (hαv²). In addition, when the flow path cross-sectional area of the flow path is constant, the quantity of air Q (m³/sec) fed into the water is proportional to the flow rate v of the air (m/sec) (Qαv).

[0081] Namely,

hαv²αQ²

[0082] and air quantity Q increases by the square root relative to increases in mean negative pressure h.

[0083] Qα{square root}h   (3)

[0084] [Relationship Between Air Quantity Q (m³/sec) and Vessel Velocity V (m/sec)]

[0085] The following results if hypothesized that height d (m) of the negative pressure forming portion is constant, and that mean negative pressure h is proportional to the flow rate V′ (m/sec) of gas-liquid two-phase flow in the vicinity of the air outlet (around the AIP):

hαV′²   (4)

[0086] Therefore,

QαV′  (5)

[0087] In addition, since flow rate V′ (m/sec) of gas-liquid two-phase flow in the vicinity of the air outlet is considered to be nearly proportional to vessel velocity V (m/sec), the following results from equation (5):

QαV′αV   (5′)

[0088] [Relationship Between Amount of Reduction of Frictional Resistance ΔF (kgf) and Air Quantity Q (m³/sec)]

[0089] The following is considered to be valid based on the findings thus far:

ΔCFαΔF/V²αQ/V

[0090] From this, the following can be derived:

ΔFαQV   (6)

[0091] In addition, the following can be derived from equations (5) and (6):

ΔFαV²   (7)

[0092] On the other hand, if it is assumed that air quantity Q(m³/sec) is nearly constant, then the following equation can be derived from equation (6):

ΔFαV   (8)

[0093] [Frictional Resistance Reduction Effect Relative to Change in Height d (m) of Negative Pressure Forming Portion when Vessel Velocity V (m/sec) is Constant (Net Gain)]

[0094] In the following example, the width of the negative pressure forming portion in the direction of vessel width is constant, and the surface area of the region opposing the flow of water is proportional to its height d. At this time, as shown in equation (9), air quantity Q (m3/sec) is proportional to the product of the flow rate v (m/sec) of the air in the flow path and the height d of the negative pressure forming portion.

QαvdαVdα{square root}hd   (9)

[0095] Furthermore, although the negative pressure region changes accompanying changes in height d of the negative pressure forming portion, that change is ignored here. The following equation is derived from equations (6) and (9) when vessel velocity V is constant:

ΔFαd   (10)

[0096] Moreover, since the width of the negative pressure forming portion is constant, the amount of increase in resistance ΔR (kgf) is proportional to height d of the negative pressure forming portion.

ΔRαd   (11)

[0097] In other words, when the width of the negative pressure forming portion is constant, if the value of ΔNV=ΔF−ΔR reaches a maximum, maximum frictional resistance reduction effects are obtained. Here, when the following equations are used:

ΔF=βd

ΔR=αd,

[0098] then the following equation results:

ΔNV=ΔF−ΔR=βd−αd

[0099] Moreover, the following equation results if ΔNV=y and d=x:

y=(β−α)x   (12)

[0100] and when y≧0, a net gain is obtained. However, the maximum value of height d of the negative pressure forming portion is not determined from equation (12).

[0101] [Frictional Resistance Reduction Effects Relative to Changes in Vessel Velocity V (m/sec) when Height d (m) of the Negative Pressure Forming Portion is Constant (Net Gain)]

[0102] The following equation is derived from equations (7) and (8):

ΔF=γV ² +δV   (13)

[0103] In addition, when height d of the negative pressure forming portion is constant, the amount of the increase in resistance ΔR is proportion to the square of vessel velocity V.

ΔR=εV²   (14)

[0104] The following equation is derived from equations (13) and (14):

ΔNd=ΔF−ΔR=(γ−ε)V ² +δV   (15)

[0105] Moreover, the following equation results when ΔNd=y, V=x, A=γ−ε and B=δ:

y=Ax ² +Bx   (16)

[0106] Here, as is clear from the derivation process, parameter α and parameter β in equation (12) are dependent on the shapes of each constituent for forming the negative pressure region (protruding shape of the negative pressure forming portion, opening surface area of the air outlet, flow path cross-sectional surface area of the flow path and so forth). In addition, γ and ε that compose parameter A in equation (16) are dependent on the characteristics of the hull (such as its output and hull shape), while δ that composes parameter B is dependent on the state of each constituent for forming the negative pressure region.

[0107] In other words, frictional resistance reduction effects fluctuate according to each of the parameters of equations (12) and (16), the protruding height d of the negative pressure forming portion and vessel velocity V. Thus, as in the manner of the frictional resistance reducing vessel as claimed in the present invention, by changing at least one of the states of each constituent consisting of the protruding state of the negative pressure forming portion from the submerged surface, the opening surface area of the air outlet, and cross-sectional surface area of the flow path, the state of the negative pressure region in the water can be changed according to the characteristics of the hull, thereby making it possible to realize effective reduction of frictional resistance. Moreover, by controlling a drive mechanism based on changes in vessel velocity with a control apparatus, the state of the negative pressure region in the water can be controlled so as to maximize frictional resistance reduction effects.

EMBODIMENT 1

[0108] Next, the following provides an explanation of an embodiment of the frictional resistance reducing vessel as claimed in the present invention with reference to the drawings.

[0109] In FIG. 2A, reference symbol M indicates a frictional resistance reducing vessel, 30 a hull, 31 a bubble generator, 32 a hull shell plate (submerged surface), 33 a propeller, and 34 a rudder.

[0110] Frictional resistance reducing vessel M is a large ship in the manner of, for example, a Very Large Crude Oil Carrier (VLCC). In comparison with other types of vessels, the surface area of the bottom of a large ship in hull shell plate 32 below draft line 15 is formed to be large relative to the side of the vessel. Furthermore, the type of vessel to which the present invention is applied is not limited to this large ship, but the hull may also be of another form such as that of a high-speed vessel or fishing vessel.

[0111] As shown in FIG. 2B, bubble generator 31 is equipped with air induction pipe 40, which is arranged inside opening 32 a provided in the bottom of the vessel, drive mechanism 41, which drives air induction pipe 40 while supporting it so as to move freely in the vertical direction, and control apparatus 42, which controls drive mechanism 41.

[0112] Air induction pipe 40 is composed mainly of a cylindrical member, and has inside a space that serves as a flow path (flow path 43). The lower end of air induction pipe 40 is in the form of inclined surface 44, which is inclined relative to the axial direction, and the lower end of flow path 43 opens into the water via air outlet 45 provided in this inclined surface 44 and facing toward the rear (stern). On the other hand, the upper end of flow path 43 opens into a gaseous space (atmosphere) via air intake port 40 a.

[0113] Cross-sectional shapes of air induction pipe 40 are shown in FIGS. 3A through 3C. In frictional resistance reducing vessel M of the present embodiment, a plurality of types of air induction pipe 40 having different cross-sectional shapes are used selectively, examples of which include the round cylindrical shape shown in FIG. 3A, the square cylindrical shape shown in FIG. 3B, and the semi-circular cylindrical shape shown in FIG. 3C.

[0114] Returning to FIG. 2, drive mechanism 41 is equipped with drive motor 47, cylindrical housing pipe 48, which guides air induction pipe 40 in the vertical direction, and a transmission unit not shown which moves air induction pipe 40 up and down inner cylindrical housing 48 by transferring the drive power of drive motor 47. Furthermore, a rack and pinion mechanism, direct drive mechanism using a linear guide and so forth are applied for the transmission unit. In addition, the upper end of housing pipe 48 is preferably arranged so that it is positioned above the draft line (water level 15). Moreover, it is preferable to provide an attachment adapter that matches each of the cross-sectional shapes of air induction pipe 40 in order to selectively house a plurality of types of air induction pipes 40 inside housing pipe 48.

[0115] Control apparatus 42 comprehensively controls the entire hull, and is composed of a microcomputer (or mini-computer) containing a central processing unit (CPU), read-only memory (ROM), random access memory (RAM) and so forth.

[0116] Next, an explanation is provided for the method of reducing the frictional resistance of hull 30 by a negative pressure method in the frictional resistance reducing vessel M having the above structure.

[0117] When the vessel is stopped, water (sea water) enters flow path 43 to nearly the same water level as that surrounding hull 30. When hull 30 enters the operating state due to the thrust of propeller 33, as shown in FIG. 4A, relative water flow 50 is formed relative to hull 30.

[0118] When hull 30 reaches a prescribed vessel velocity, as shown in FIG. 4B, control apparatus 42 adjusts the position (height) of air induction pipe 40 using drive mechanism 41, and causes lateral surface 40 b (negative pressure forming portion) of air induction pipe 40 to protrude by a prescribed height d from hull shell plate 32.

[0119] At this time, the water flow path is narrowed by lateral surface 40 b of air induction pipe 40. As a result, together with the flow rate of water flowing along hull shell plate 32 increasing, a breakaway region is formed in the water due to the sharp angle of the protruding end of air induction pipe 40. Moreover, due to the decrease in pressure accompanying increased flow rate (based on Bernoulli's theorem), and the breakaway action and cavitation in the breakaway region, hydrostatic pressure decreases locally in the water on the side of inclined surface 44 of air induction pipe 40, resulting in the formation of negative pressure region 51 that is at low pressure relative to the atmosphere.

[0120] At this time, due to the pressure of air outlet 45 facing negative pressure region 51 being lower than the pressure at air intake port 40 a, pressure gradient force acts on the fluid (sea water and air) in flow path 43, which together with causing sea water to be discharged from flow path 43, air that has entered from air intake port 40 a is fed into the water from flow path 43.

[0121] Air that has been fed into the water mixes into the water in the form of microbubbles 52, and the frictional resistance of hull 30 is reduced due to the presence of a large number of bubbles 52 in the vicinity of hull shell plate 32.

[0122] Here, as was previously mentioned, the energy required for feeding air into the water is obtained by changing the flow state of the water by air induction pipe 40, and this energy is less than a conventional positive pressure method. Consequently, in the frictional resistance reducing vessel M of the present embodiment which uses a negative pressure method, bubbles are generated in the water with less energy consumption than conventional positive pressure methods, thereby enabling frictional resistance during operation to be efficiently reduced.

[0123] Moreover, as was previously described, frictional resistance reduction effects fluctuate according to the characteristics of hull 30 (such as output and hull shape), the shape of air induction pipe 40 that forms negative pressure region 51 (such as the protruding shape of air induction pipe 40 from hull shell plate 32, the opening surface area of air outlet 45, and the surface area of flow path 43), and vessel velocity. Therefore, in the frictional resistance reducing vessel M as claimed in the present invention, at least one of the shape of air induction pipe 40, the protruding height of air induction pipe 40 from hull shell plate 32 and vessel velocity is changed by drive mechanism 41 so that maximum frictional resistance reduction effects are achieved. FIG. 5 is a flow chart showing an example of this procedure.

[0124] As shown in this flow chart, the shape of air induction pipe 40 is first selected (Step 100). For example, the shape of air induction pipe 40 that is effective for reducing frictional resistance is determined by selectively using a plurality of air induction pipes 40 having different shapes and comparing the changes in vessel velocity at those times.

[0125] More specifically, air induction pipes 40 having different cross-sectional shapes as shown in, for example, FIGS. 3A through 3C are sequentially installed on drive mechanism 41 and the ends (lower end) of air induction pipes 40 are protruded by the same height d from hull shell plate 32 either manually or by a drive mechanism not shown with the rotating speed of propeller 33 constant. At this time, since the opening surface area of air outlet 45 and the surface area of flow path 43 differ according to the shape of the air induction pipe 40 that is installed, the state of negative pressure region 51 changes. Therefore, the changes in vessel velocity (increases or decreases) at that time are compared, and the air induction pipe 40 for which vessel velocity increased the most is selected based on the results of that comparison. Furthermore, although three types of air induction pipes 40 having mutually different cross-sectional shapes are shown in FIGS. 3A through 3C, the shape of air induction pipe 40 is not limited to these, but rather the shape and number of air induction pipes that can be selected are random.

[0126] When the shape of air induction pipe 40 has been selected, the optimum protruding state of air induction pipe 40 is determined by control apparatus 42 (Steps 101 and 102). More specifically, control apparatus 42 controls drive mechanism 41 to change the protruding height d of air induction pipe 40 from hull shell plate 32 with the rotating speed of propeller 33 constant (for example, at the rotating speed corresponding to the standard cruising velocity). The optimum protruding height d of air induction pipe 40 relative to a prescribed vessel velocity is then determined from the change (increase or decrease) in vessel velocity at this time (Step 101). Furthermore, the optimum protruding height d may also be determined based on the above-mentioned equation (12) by resolving parameters β and α in equation (12) using the method of least squares and so forth and calculating that solution.

[0127] In addition, control apparatus 42 determines the optimum vessel velocity relative to the prescribed protruding height d from the change (increase or decrease) in vessel velocity when the rotating speed of propeller 33 is changed with the protruding height d of air induction pipe 40 (for example, the protruding height determined in Step 102) constant. Furthermore, the optimum vessel velocity may also be determined based on the above-mentioned equation (16) by resolving parameters A and B in equation (16) and calculating that solution based on data on the increase or decrease in vessel velocity when the rotating speed of propeller 33 (vessel velocity) is changed.

[0128] The closer the optimum vessel velocity calculated at this time is to the current vessel velocity, the more effective the current protruding height d is on reducing frictional resistance. Thus, control apparatus 42 repeatedly changes protruding height d of air induction pipe 40 and vessel velocity within a prescribed range to search for the maximum increase in vessel velocity (frictional resistance reduction effect: ΔN) (Step 103). As a result, air induction pipe 40 is controlled at the optimum protruding state relative to a desired vessel velocity.

[0129] In this manner, in the frictional resistance reducing vessel M of the present embodiment, by changing the shape of air induction pipe 40 (cross-sectional shape and protruding height from hull shell plate 32) based on the change in vessel velocity, the state of negative pressure region 51 in the water can be controlled so as to maximum frictional resistance reduction effects for a desired vessel velocity. Thus, the frictional resistance of hull 30 can be effectively reduced by juxtaposition of a large number of bubbles on hull shell plate 32 while consuming little energy, thereby making it possible to effectively conserve energy consumed during vessel operation.

[0130] Moreover, since bubble generator 31 employs a simple composition and eliminates the need for an apparatus for pressurizing the gas, it goes without saying that the construction cost of hull 30 can be held to a low level.

[0131] The size, number and location of bubble generator 31 should be determined based on the results of flow field analysis and operational testing, etc. obtained by computational fluid dynamics (CFD) so that the flow of water in the vicinity of opening 32 a of hull shell plate 32 during operation is of the desired state.

[0132] In addition, the method of changing the state of the negative pressure region in the water is not limited to the method explained in the above embodiment, but rather should be changed by changing at least one of the protruding state of the negative pressure forming portion (lateral surface 40 b of air induction pipe 40) from hull shell plate 32, the opening surface area of air outlet 45, or the cross-sectional surface area of flow path 43. Moreover, in the above embodiment, although the opening surface area of air outlet 45 and cross-sectional surface area of flow path 43 are changed by selectively using a plurality of air induction pipes 40 having different shapes, an opening and closing mechanism that changes the cross-sectional surface area of flow path 43 may be provided, and this may be driven according to changes in vessel velocity.

[0133] In addition, in the above embodiment, since the end of air induction pipe 40 having an apical shape is made to protrude from hull shell plate 32, cavitation occurs easily behind it. This results in the advantages that, when this cavitation occurs, gas and water are aggressively mixed at the interface between the gas and water due to its agitation effects, which together with promoting the release of bubbles 52 from the gas-liquid interface, causes a large amount of gas to be introduced into the water via flow path 43 due to the strong negative pressure action generated by cavitation, resulting in a large number of bubbles 52 being mixed into the water. However, the shape of air induction pipe 40 is not limited to that shown in FIG. 2B, but rather is arbitrarily determined so that a negative pressure region is effectively formed in the water. In addition, with respect to the shape of air induction pipe 40, the cross-sectional area of flow path 43 is preferably as large as possible so as to take in as much air as possible, and the resistance added by lateral surface 40 a as the negative pressure forming portion that protrudes from hull shell plate 32 is preferably as low as possible.

EMBODIMENT 2

[0134] The following provides an explanation of another embodiment of the frictional resistance reducing vessel as claimed in the present invention with reference to the drawings. FIG. 6A shows an example of arranging bubble generator 111, instead of bubble generator 31, on the bottom of frictional resistance reducing vessel M shown in FIG. 2A.

[0135] As shown in FIG. 6B, this bubble generator 111 is equipped with outer cylinder 121, which extends in the vertical direction and is fixed to hull 30, inner cylinder 122 in the form of an air induction pipe (AIP) that is housed within outer cylinder 121 while being able to be attached and removed and being able to move freely along the axial direction of outer cylinder 121 (vertical direction), negative pressure forming portion 123 provided on the lower end of inner cylinder 122, and position adjustment unit 124 for adjusting the position (height) in the axial direction of inner cylinder 122 relative to outer cylinder 121.

[0136] Outer cylinder 121 has a cylindrical shape, and is installed by passing through hull 30 so that both ends are open above and below draft line 15. Inner cylinder 122 is a cylindrically shaped member of a size that is able to be inserted into outer cylinder 121, and is inserted into outer cylinder 121 through an opening formed in the upper end of outer cylinder 121. Although corrosion-resistance surface-treated steel or aluminum and so forth is used for the material of outer cylinder 121 and inner cylinder 122, resin having corrosion resistance relative to sea water (synthetic resin) is preferably used for the material of inner cylinder 122 for the purpose of reducing weight.

[0137] Negative pressure forming portion 123 has a box shape in which one face is open, and the open end is coupled while maintaining airtightness to the lower end of inner cylinder 122 so that a projection is formed from the lower end of inner cylinder 122 in the downward direction. More specifically, negative pressure forming portion 123 has forward inclined surface 123 a, which extends on an incline relative to the axial direction of inner cylinder 122 and faces toward the front (bow) of the direction of progress Dv, and backward inclined surface 123 b positioned on its back side and facing toward the rear (stern) of the direction of progress. A roughly apically shaped projection is formed that protrudes from hull shell plate 32 perpendicularly relative to the direction of progress Dv of hull 30 by mutually joining the edges of these inclined surfaces 123 a and 123 b. Furthermore, discharge outlet 123 c, which is continuous with the opening formed in the lower end of inner cylinder 122, is provided in backward inclined surface 123 b.

[0138] In addition, as a result of inner cylinder 122 being arranged inside outer cylinder 121, flow path 130 is formed inside inner cylinder 122 and negative pressure forming portion 123. In addition to one end of flow path 130 opening into a gaseous space (atmosphere) via opening 122 a formed in the form of an air intake port in the upper end of inner cylinder 122, the lower end of flow path 130 opens into the water via discharge outlet 123 c of negative pressure forming portion 123.

[0139] Position adjustment unit 124 is equipped with a drive apparatus not shown such as a motor for moving inner cylinder 122 to a prescribed position, and a locking mechanism and so forth not shown for locking inner cylinder 122 at that prescribed position, and causes negative pressure forming portion 123 to protrude from hull shell plate 32 to a prescribed position corresponding to the operating state of hull 30.

[0140] The shape and locations of each composite member of bubble generator 111 are designed based on the results of flow field analysis and operational testing, etc. obtained by computational fluid dynamics so that the flow of water behind (on the stern side) of negative pressure forming portion 123 during operation is of the desired state. Here, the height H of negative pressure forming portion 123 is determined so that cavitation and separation occur in the water behind negative pressure forming portion 123 itself due to the flow of water relative to hull 30. For example, height H of negative pressure forming portion is set to be as large as possible within the range of diameter D₁ of inner cylinder 122. Furthermore, one or a plurality of bubble generator 111 are arranged according to the size of the vessel bottom.

[0141] The following provides an explanation of a method of reducing the frictional resistance of hull 30 by the frictional resistance reducing vessel having the structure described above with reference to FIGS. 7A and 7B.

[0142] When the vessel is stopped, water (sea water) enters flow path 130 to nearly the same water level as that surrounding hull 30. When hull 30 enters the operating state due to the thrust of propeller 33, relative water flow 140 is formed relative to hull 30.

[0143] When a prescribed vessel velocity V is reached in the operating state, as shown in FIG. 7A, positioning adjusting unit 124 adjusts the position (height) in the axial direction of inner cylinder 122 relative to outer cylinder 121 so that negative pressure forming portion 123 protrudes by a prescribed height H from hull shell plate 32.

[0144] At this time, the water flow path is narrowed by forward inclined surface 123 a of negative pressure forming portion 123. As a result, together with the flow rate of water flowing along the vessel bottom increasing, cavitation and separation occur in the water behind negative pressure forming portion 123 due to the sharp angle of its protruding end. Consequently, hydrostatic pressure decreases locally in the water behind negative pressure forming portion 123, and negative pressure region 141 is formed at a lower pressure than the atmosphere. At this time, since the pressure of outlet port 123 c facing negative pressure region 141 is lower than the pressure at opening 122 a formed in the upper end of inner cylinder 122, pressure gradient force acts on the fluid (sea water and air) in fluid path 130, which together with causing sea water to be discharged from flow path 130, air that has entered from opening 122 a formed in the upper end of inner cylinder 122 is fed into the water by flowing through flow path 130.

[0145] Air that has been fed into the water mixes into the water in the form of bubbles 142, and as a result, the frictional resistance of hull 30 is reduced due to the presence of a large number of bubbles 142 in the vicinity of hull shell plate 32.

[0146] At this time, the energy required for feeding air into the water is mainly the energy for changing the location of the gas. This energy is obtained by changing the flow state of the water by negative pressure forming portion 123, and this energy is less than the energy consumed in the case of pressurizing a gas and blowing it into the water. Namely, according to the present invention, energy consumption during vessel operation is effectively conserved by reducing the frictional resistance of hull 30 with a lower level of energy consumption.

[0147] In addition, in the present embodiment, negative pressure forming portion 123 in the form of a roughly apically shaped projection is arranged protruding from hull shell plate 32, and aggressively forms cavitation and separation. Consequently, due to the resulting agitation effects, a large amount of gas is introduced into the water via flow path 130 resulting in the generation of a large number of bubbles. At this time, although drag relative to water flow 140 increases since negative pressure forming portion 123 is arranged protruding from hull shell plate 32, the Reynold's number on an actual hull 30 is comparatively high due to the surface roughness of hull shell plate 32. Moreover, since a large number of bubbles are generated in the water, the increase in drag has little effect on frictional resistance reduction effects.

[0148] Moreover, in the present embodiment, the protruding state of negative pressure forming portion 123 from hull shell plate 32 is controlled by adjusting the position (height) of inner cylinder 122 relative to outer cylinder 121 according to the vessel's operating state. Thus, in the case, for example, the vessel has not reached a prescribed vessel velocity V, or in the case frictional resistance reduction effects produced by bubbles cannot be expected due to inclement weather, as shown in FIG. 7B, by moving negative pressure forming portion 123 to the inside of hull shell plate 32 by position adjustment unit 124 so that negative pressure forming portion 123 is no longer protruding, the increase in drag relative to water flow 140 is suppressed making it possible to reduce energy consumption. Moreover, by adjusting the protruding height of negative pressure forming portion 123 according to the operating velocity, bubbles 142 can be controlled to be effectively released into the water.

[0149] The main governing factors for the formation of negative pressure region 141 are the shape of negative pressure forming portion 123, its protruding height from hull shell plate 32, and the Reynold's number. Since the present invention is not considered to be susceptible to disadvantages caused by water depth, the technology as claimed in the present invention is also advantageous for application to large ships.

[0150] In addition, since bubble generator 111 has a simple composition and eliminates the need for an apparatus for pressurizing the gas, it goes without saying that the construction cost of hull 30 can be held to a low level.

[0151] Furthermore, the various shapes and combinations, etc. of each composite member shown in the embodiment described above refer to only a single example, and can be altered in various ways based on design requirements and so forth within a range that does not deviate from the purport of the present invention. In addition, although the above embodiment indicated an example of applying the present invention to a large ship, the present invention is not limited to large ships, but can also be applied to other vessels such as high-speed vessels and fishing vessels. Furthermore, the size, number and location of bubble generator 111 should be suitably determined according to the shape of hull 30. However, as was previously mentioned, since the Reynold's number on an actual hull 30 is already comparatively high due to the surface roughness of hull shell plate 32 resulting in the action of diffusion effects, it is not necessary to provide an excessively large number of bubble generators 111.

EMBODIMENT 3

[0152] The following provides an explanation of still another embodiment of the frictional resistance reducing vessel as claimed in the present invention with reference to the drawings. FIG. 9A is a side view of the starboard side in the vicinity of the bow of frictional resistance reducing vessel M shown in FIG. 2A that shows an example in which bubble generator 201 is arranged on the bottom of frictional resistance reducing vessel M shown instead of bubble generator 31. In addition, FIG. 9B is an overhead view as viewed from the bottom of vessel M, while FIG. 9C is a cross-sectional view taken along line A-A in FIG. 9A. In these drawings, reference symbol 202 is an outer cylinder, 203 an inner cylinder, 204 a stopping mechanism (protrusion adjustment means) and 205 a cover. Among these, outer cylinder 202, inner cylinder 203 and stopping mechanism 204 compose bubble generator 201 in frictional resistance reducing vessel M.

[0153] Outer cylinder 202 is a through path that passes vertically through the inside of hull 30, its upper end (one end) 202 a opens to the atmosphere, and its lower end (other end) 202 b is welded to hull 30 so as to open into the water. The cross-sectional shape of this outer cylinder 202 is an asymmetrical shape in the lengthwise direction L of frictional resistance reducing vessel M consisting of, as shown in FIG. 9C for example, an isosceles triangle of which the bottom side a is perpendicular to lengthwise direction L. Inner cylinder 203, having an outer diameter slightly smaller than the inner diameter of outer cylinder 202, is inserted into this outer cylinder 202. The cross-sectional shape of at least the portion of this inner cylinder 203 that protrudes into the water is in the form of an isosceles triangle having an outer diameter slightly smaller than outer cylinder 202 so that inner cylinder 203 is able to freely move vertically within outer cylinder 202.

[0154] Upper end (one end) 203 a of inner cylinder 203 is open to the atmosphere, while air spraying portion 206 is provided in the lower end (other end) 203 b of inner cylinder 203. As shown in FIG. 9B, together with obstructing lower end 203 b of inner cylinder 203, this air spraying portion 206 is composed of isosceles triangle face plate 206 b in which circular spray outlet 206 a is formed in its center, and half-dome-shaped protruding plate 206 c that protrudes from face plate 206 b and covers spray outlet 206 a from the front (bow side) of hull 30.

[0155] Namely, in the case of setting to a position along the vertical direction of inner cylinder 203 so that face plate 206 b is located roughly in the same plane as hull shell plate 32, this air spraying portion 206 protrudes from hull shell plate 32. In addition, since half-dome-shaped projecting plate 206 c covers the front half of spray outlet 206 a from the front side of hull 30, the protruding shape of air spraying portion 206 is asymmetrical in lengthwise direction L of hull 30.

[0156] In addition, inner cylinder 203 installed in outer cylinder 202 is stopped and prevented from moving further by stopping mechanism 204, and the vertical position of inner cylinder 203 relative to outer cylinder 202 is set by stopping mechanism 204. Stopping mechanism 204 is able to stop inner cylinder 203 inside outer cylinder 202 at least in the state in which face plate 206 b and hull shell plate 32 are roughly in the same plane (State A), and in the state in which protruding plate 206 c is not protruding from hull shell plate 32 as shown in FIG. 9A.

[0157] In addition, cover 205 occludes upper end 202 a of outer cylinder 202 open to the atmosphere when frictional resistance reducing vessel M is stopped, and prevents, for example, water from being blown up through outer cylinder 202 during inclement weather and so forth.

[0158] The following provides an explanation of the method of reducing frictional resistance of hull 30 by frictional resistance reducing vessel 30 having the structure described above.

[0159] In the case of this frictional resistance reducing vessel M, inner cylinder 203 is set in state B when the vessel is stopped. At this time, sea water enters inner cylinder 203 to the same height (water level) as draft line 15. In the operating state, inner cylinder 203 is set in state A. When the vessel is operating, atmosphere (air) that has been taken in from upper end 203 a of inner cylinder 203 is sprayed into the water from air spraying portion 206, and this air is dispersed in the direction of the stern in the form of microbubbles by the flow of water (turbulent flow).

[0160] Namely, in the case frictional resistance reducing vessel M is operating at cruising velocity in state A, the flow path of water that flows from the direction of the bow to the stern in the vicinity of air spraying portion 206 is locally narrowed by the presence of protruding plate 206 c. As a result, the static pressure of water at air blowing portion 206, or in other words at lower end 203 b of inner cylinder 203, decreases below atmospheric pressure, or in other words the pressure at upper end 203 a of inner cylinder 203, resulting in a state of negative pressure. As a result, air taken in from upper end 203 a is sprayed into the water from spray outlet 206 a. Air that has been sprayed into the water in this manner takes on the form of bubbles as a result of being finely dispersed by water flow from the bow to the stern. Bubbles generated in the vicinity of the bow are sequentially dispersed and move towards the stern along the lines of water flow and broadly cover the vessel's bottom, thereby reducing the frictional resistance between hull shell plate 32 and the water at the vessel's bottom.

[0161] In addition, together with the cross-sectional shape of inner cylinder 203, which is installed so as to engage while moving freely with outer cylinder 202, being in the form of an isosceles triangle, since the vertical position of inner cylinder 203 can be changed by stopping mechanism 204, inspection and maintenance of bubble generator 201 as well as testing of friction reduction effects and so forth can be performed easily.

[0162] Moreover, since the cross-sectional shapes of outer cylinder 202 and inner cylinder 203 are set to the shape of an isosceles triangle, in the case inner cylinder 203 is extracted from outer cylinder 202 during maintenance and inspection, the orientation of air spraying portion 206, which has an asymmetrically protruding shape in the lengthwise direction of hull 30, can be made to be the normal orientation, namely the orientation such that half-dome-shaped protruding plate 206 c covers spray outlet 206 a from the front side of hull 30, simply inserting inner cylinder 203 into outer cylinder 202 without paying attention to the orientation of air spraying portion 206. Thus, installation of inner cylinder 203 in outer cylinder 202 during maintenance, inspections and so forth can be performed easily as compared with the case of the cross-sectional shapes of outer cylinder 202 and inner cylinder 203 having a symmetrical shape in the lengthwise direction L (such as a circular or oval shape).

[0163] In addition, the vertical position of inner cylinder 203 can be changed between state A and state B by stopping mechanism 204 in this frictional resistance reducing vessel M, differences in frictional reduction effects due to the presence or absence of protrusion of protruding plate 206 c relative to hull shell plate 32, namely due to the presence or absence of the spraying of air into the water, can be tested easily.

[0164] Furthermore, the present invention is not limited to the above embodiment, but also can be modified, for example, in the manner described below.

[0165] (1) Although the cross-sectional shapes of outer cylinder 202 and inner cylinder 203 were in the form of an isosceles triangle in the above embodiment, air spraying portion 206 is always at the normal orientation when inner cylinder 203 is inserted into outer cylinder 202 if they have an asymmetrical shape in the lengthwise direction of frictional resistance reducing vessel M. Thus, the above cross-sectional shape may be another shape such as a pentagon or heptagon. However, an excessively complex shape is undesirable in terms of production cost. From the viewpoint of production cost, the cross-sectional shape of outer cylinder 202 and inner cylinder 203 is preferably an isosceles triangle, which is asymmetrical in lengthwise direction L of frictional resistance reducing vessel M, and is also the simplest shape.

[0166] (2) Although half-dome-shaped projecting plate 206 c is provided over face plate 206 b so as to cover roughly half of spray outlet 206 a in air spraying portion 206 of the above embodiment, the present invention is not limited to this. Protruding plate 206 c may also be provided so as to cover less than half or more than half of spray outlet 206 a. However, the extent to which protruding plate 206 c covers spray outlet 206 a is, as a general rule, determined from the viewpoint of whether or not spray outlet 206 a most efficiently forms a negative pressure state.

EMBODIMENT 4

[0167] The following provides an explanation of still another embodiment of the frictional resistance reducing vessel as claimed in the present invention with reference to the drawings. In FIG. 10A, reference symbol Ma indicates a frictional resistance reducing vessel, 60 a hull, 62 a hull shell plate (submerged surface), 63 a propeller, 64 a rudder and 311 a bubble generator.

[0168] As shown in FIG. 10B, bubble generator 311 is equipped with indentation 320 formed so as to be recessed from hull shell plate 62 of the vessel bottom, flow path 321 which passes through hull 60 and is open above and below draft line 15, and negative pressure forming member 322 arranged inside indentation 320.

[0169] Indentation 320 is formed by chamber 330 attached to hull shell plate 62 from the inside of hull 60. Namely, chamber 330 is formed into the shape of a box of which one face is open, and that open end is connected to hull shell plate 62 from the inside of hull 60. Furthermore, chamber 330 is comprised of an integrally molded article made of hard polyurethane, and is removably attached to hull shell plate 62.

[0170] Flow path 321 is a space formed inside air induction pipe (AIP) 331 connected to chamber 330. Namely, discharge outlet 330 a, for releasing bubbles into the water, is located in the back of indentation 320 in chamber 330, and air induction pipe 331 comprised of a tubular member is connected to discharge outlet 330 a. As a result, one end of flow path 321 is open to a gaseous space (atmosphere) via air intake port 331 a of air induction pipe 331, while the other end is open into the water via discharge outlet 330 a of the above chamber 330. In addition, the internal cross-sectional area and shape of air induction pipe 331 are determined so that a desired flow volume of liquid flows through flow path 321 with low pressure loss.

[0171] Negative pressure forming member 322 is arranged so that at least a portion protrudes from hull shell plate 62, forms a negative pressure region in the water behind itself (on the stern side) that is at a lower pressure than the gaseous space (atmosphere) at a prescribed vessel velocity Vs by using the relative flow of water with respect to hull 60 during vessel operation, and is supported while allowing to rotate freely centering around stainless steel support shaft 332.

[0172] In addition, negative pressure forming member 322 is comprised of a member in which the cross-sectional shape in the direction perpendicular to support shaft 332 is roughly that of an isosceles triangle, and has a plurality (here, three) of faces 322 a, 322 b and 322 c parallel to support shaft 332. Moreover, negative pressure forming member 322 is at a prescribed angle centering around support shaft 332 due to the action of angle adjustment mechanism 323 that controls the rotation of support shaft 332. As a result, a portion of prescribed faces 322 a and 322 b protrude from hull shell plate 62, and from a different angle, prescribed face 322 a is positioned roughly in the same plane as hull shell plate 62. Furthermore, negative pressure forming member 322 is comprised of a molded article made of hard polyurethane.

[0173] As shown in, for example, FIG. 12, angle adjustment mechanism 323 is equipped with bearings 333 and 334, which support support shaft 332 while allowing to rotate freely and which are supported in chamber 330, levers 335 and 336 fixed to support shaft 332 on the outside of chamber 330 for assisting in the rotation of support shaft 332, and locking portion 337 for locking the rotation of support shaft 332. In this example, the arrangement angle of negative pressure forming member 332 is changed centered about support shaft 332 by an operator rotating support shaft 332 by means of levers 335 and 336, and the arrangement angle of negative pressure forming member 332 can be set by locking support shaft 332 with locking portion 337.

[0174] Furthermore, angle adjustment mechanism 323 is not limited to the structure by which the arrangement angle of negative pressure forming member 322 is adjusted by the above manual operation, but rather a structure may also be employed in which, for example, the arrangement angle of negative pressure forming member 322 is driven automatically by having a driving motor and so forth. Furthermore, bearings 333 and 334 have a sufficiently sealed structure so as to prevent the entrance of water into the hull from the water.

[0175] In addition, the shapes and locations of each constituent member of bubble generator 311 are designed based on the results of flow field analysis and operational testing, etc. obtained by computational fluid dynamics so that the flow of water behind (stern side) of negative pressure forming member 322 during operation is of the desired state. For example, the height of negative pressure forming member 322 and the shape of chamber 330 are determined so that a negative pressure region is formed in the water behind negative pressure forming member 322 that is at a lower pressure than the gaseous space (atmosphere) during operation at prescribed vessel velocity Vs.

[0176] Furthermore, in addition to the hard polyurethane previously mentioned, a material such as corrosion-resistant metal or plastic that has surface corrosion resistance primarily with respect to sea water and is also resistant to adherence to the surface by marine organisms is preferably used for the material of negative pressure forming member 322, chamber 330 and air induction pipe 331. In addition, one or a plurality of bubble generators 311 may be arranged corresponding to the size of the vessel bottom.

[0177] The following provides an explanation of the method of reducing frictional resistance of hull 60 by frictional resistance reducing vessel Ma having the above structure with reference to FIGS. 11A and 11B.

[0178] When the vessel is stopped, water (sea water) enters flow path 321 to roughly the same water level as that surrounding hull 60. When hull 60 enters the operating state due to the thrust of propeller 63, relative water flow 340 is formed with respect to hull 60. When the vessel reaches a prescribed velocity V, as shown in FIG. 11A, by adjusting the angle of negative pressure forming member 322 by angle adjustment mechanism 323, a portion of negative pressure forming member 322, namely a portion of prescribed faces 322 a and 322 b of negative pressure forming member 322, protrudes from hull shell plate 62.

[0179] At this time, as a result of the water flow path being narrowed by face 322 a of negative pressure forming member 322, together with the flow rate of water flowing along the vessel bottom increasing, due to the sharp angle of its protruding end, a separation region is formed in the water, and due to the presence of this separation region, hydrostatic pressure in the water behind face 322 a of negative pressure forming member 322 locally decreases, resulting in the formation of negative pressure region 341 at a lower pressure relative to the atmosphere.

[0180] This being the case, the pressure of discharge outlet 330 a facing negative pressure region 341 becomes lower than the pressure at air intake port 331 a. As a result, a pressure gradient acts on the fluid (sea water and air) inside flow path 321, and together with sea water being discharged from flow path 321, air that has entered from air intake port 331 a is fed into the water via flow path 321.

[0181] Gas that has been fed into the water mixes into the water in the form of bubbles 342 after being released from gas-liquid interface 343, and the frictional resistance of hull 60 is reduced due to juxtaposition of a large number of bubbles 342 in the vicinity of hull shell plate 62.

[0182] At this time, the energy required to feed air into the water is mainly the energy for changing the location of the gas. This energy is obtained by changing the flow state of the water by negative pressure forming member 322, and is lower than the energy consumed in the case of pressuring a gas and spraying it into the water. Namely, according to the present embodiment, energy consumption during vessel operation is effectively conserved by reducing the frictional resistance of hull 60 with a low level of energy consumption.

[0183] In addition, in the present embodiment, the protruding state of negative pressure forming member 322 from hull shell plate 62 is controlled by adjusting the angle of negative pressure forming member 322 with angle adjustment mechanism 323 according to the vessel operating state. Namely, in the case, for example, the vessel has not reached a prescribed vessel velocity V or in the case frictional resistance reduction effects produced by the bubbles cannot be expected due to inclement weather, as shown in FIG. 11B, by adjusting the angle of negative pressure forming member 322 with angle adjustment mechanism 323, and positioning prescribed face 322 a of negative pressure forming member 322 in roughly the same plane as hull shell plate 62 so that negative pressure forming member 322 is not protruding, the increase in drag relative to water flow 340 is inhibited, thereby making it possible to reduce energy consumption. Moreover, by adjusting the protruding height of negative pressure forming member 322 according to vessel velocity, bubbles in the water are controlled so as to be effectively released.

[0184] In this manner, in the present embodiment, the frictional resistance of hull 60 can be effectively reduced by effectively releasing bubbles into the water according to operating velocity or inhibiting excessive increases in the drag of hull 60 as a result of controlling the protruding state of negative pressure forming member 322 from hull shell plate 62 according to the vessel operating state.

[0185] Moreover, since the protruding state of negative pressure forming member 322 from hull shell plate 62 is controlled by adjusting the angle of negative pressure forming section 322, bubble generator 311 can be composed simply and in a compact form, enabling bubble generator 311 to be easily additionally attached to the existing hull.

[0186] In addition, in the present embodiment, since negative pressure forming member 322 is prevented from protruding by arranging prescribed face 322 a of negative pressure forming member 322 in roughly the same plane as hull shell plate 62, the opening of indentation 320 is broadly obstructed by its face 322 a which reduces the surface irregularity in hull shell plate 62, and thereby effectively inhibits the increase in drag relative to water flow 340.

[0187] In addition, together with bubble generator 311 having a simple composition, since an apparatus for pressurizing a gas is not required, it goes without saying that the construction cost of hull 60 can be held to a low level. Moreover, since chamber 330 and negative pressure forming member 322 shown in FIG. 10B are molded articles made of hard polyurethane, it is easier to achieve cost reductions through mass production. In addition, since chamber 330 is removably attached to hull shell plate 62, less manpower is required during maintenance.

[0188] Furthermore, the various shapes and combinations, etc. of each of the constituent members shown in the above embodiment merely represent one example, and can be altered in various ways based on design requirements and so forth within a range that does not deviate from the purport of the present invention. The present invention also includes, for example, the variations described below.

[0189] Although angle adjustment mechanism 323 is operated manually based on the operating velocity and weather conditions in the above embodiment, the present invention is not limited to this, but rather angle adjustment mechanism 323 may be driven automatically on the base of operating velocity and other data so as to adjust the arrangement angle of negative pressure forming member 323.

[0190] In addition, although the above embodiment indicated an example of applying the present invention to a small fishing vessel, the present invention is not limited to this, but rather can also be applied to other vessels such as tankers, container ships and other large ships as well as high-speed vessels. Furthermore, the size, number and location of bubble generator 311 are suitably set according to the shape of the hull.

EMBODIMENT 5

[0191] The following provides an explanation of still another embodiment of the frictional resistance reducing vessel as claimed in the present invention with reference to the drawings. FIG. 13A shows an example of arranging bubble generators 411 on the bottom of frictional resistance reducing vessel M shown in FIG. 10A instead of bubble generator 311. In addition, as shown in FIG. 13B, a plurality of bubble generators 411 are arranged in a row in the direction of vessel width on the vessel bottom near the bow in the present embodiment.

[0192] As shown in FIGS. 14A and 14B, bubble generator 411 contains indentation 420 formed so as to be recessed from hull shell plate 62 of the vessel bottom, flow path 421, one end of which is open to the atmosphere, while the other end is open to the inside of indentation 420, wing body 422 having a wing and arranged inside indentation 420, and positioning mechanism 423 that supports wing body 422 while allowing to move freely along the lateral surface of indentation 420, and positions the wing in a prescribed position.

[0193] Indentation 420 is formed by chamber 430 having a rectangular shape that is coupled to hull shell plate 62 from the inside of hull 60. Namely, chamber 430 is formed into the shape of a box of which one face is open, and the open end is coupled from the inside of hull shell plate 62.

[0194] Here, flow path 421 is a space formed inside air induction pipe (AIP) 431 connected to chamber 430. Opening 430 a for introducing a gas into the water is provided in chamber 430, and air induction pipe 431 is connected to this opening 430 a. Furthermore, as shown in FIG. 14A, the installation space of air induction pipe 431 can be used more efficiently by using a flexible tubular member for air induction pipe 431.

[0195] As shown in FIG. 16, wing body 422 is mainly composed of base 435, struts 436 and 437, and wing 438. Base 435 has a chamber structure, and has inclined surface 435 a on the bottom. This inclined surface 435 a here is formed into an upward curved shape, and is formed so that its height gradually becomes higher towards the front in direction of progress Dv (bow side). The ends of each of the above struts 436 and 437 are respectively coupled to both ends in the direction of width of this inclined surface 435, and the other end of each strut 436 and 437 is respectively coupled to both ends in the direction of width of the above wing 438. Struts 436 and 437 and wing 438 are respectively formed into a prescribed wing form, and the leading and trailing edges are arranged to as to face direction of progress Dv of hull 60. In addition, wing 438 is arranged so the convex wing surface is facing upward. Furthermore, an NACA wing form, an Ozibal wing form or various other wing forms can be applied for the shapes of wing 438 and struts 436 and 437, and are set according to the shape and velocity of the hull (standard cruising velocity).

[0196] In addition, base 435, struts 436 and 437, and wing 438 are mutually assembled to form a cylindrical shape in wing body 422, and form curved water channel 439 in which a convex portion faces upward in the vertical direction on the inside. In addition, openings 435 b and 435 c are formed in the upper and lower (inclined surface 435 a) surfaces of base 435, and the above water channel 439 is connected with the space located above base 435 through these openings 435 b and 435 c.

[0197] Returning to FIG. 14A, positioning mechanism 423 is in the form of a linking mechanism that slides wing body 422 vertically along the lateral surface of indentation 420, and the height of wing 438 from hull shell plate 62 can be varied by operating lever 440. Furthermore, lever 440 is arranged on the deck, and its operating position is maintained by a stopping means such as a hook. In addition, flexible cover 441 in the form of a bellows and so forth is arranged on positioning mechanism 423 which isolates the above indentation 420 from the deck while adapting to the movement of lever 440.

[0198] The following provides an explanation of the method of reducing frictional resistance of hull 60 by frictional resistance reducing vessel Ma having the structure described above with reference to FIGS. 15A and 15B.

[0199] When the vessel is stopped, water (sea water) enters indentation 420 to about the same water level as that around hull 60. When hull 60 enters the operating state due to the thrust of propeller 63, relative water flow 450 is formed with respect to hull 60. When the vessel reaches a prescribed velocity (for example, the standard cruising velocity), as shown in FIG. 15A, wing body 422 is moved downward by positioning mechanism 423 as a result of operating lever 440, causing wing 438 to protrude to a prescribed height from hull shell plate 62. At this time, the inside of depression 420 is open to water flow 450, and a separation region is formed in the water due to the level difference of the entrance of indentation 420. Due to this separation region, water pressure (hydrostatic pressure) in water channel 439 decreases. In addition, since the flow path of water along inclined surface 435 a gradually narrows in water channel 439, the flow rate of water flowing through water channel 439 increases towards the back and water pressure decreases further. As a result, the water pressure in water channel 439 forms a locally negative pressure relative to the atmosphere.

[0200] Due to this pressure difference relative to the atmosphere, pressure gradient Pf acts on the fluid (sea water and air) inside flow path 421 and indentation 420, and air flows from the atmosphere into flow path 421 and indentation 420. This air is then fed into the water in water channel 439 via openings 435 b and 435 c of wing body 422.

[0201] Air that has been fed into the water mixed into the water in the form of bubbles 452, and the frictional resistance of hull 60 is reduced due to the juxtaposition of a large number of bubbles 452 in the vicinity of hull shell plate 62.

[0202] At this time, the energy required for feeding air into the water is mainly the energy for changing the location of the gas. Since this energy is obtained by changing the flow state of the water by indentation 420 and wing body 422, this energy is less than the energy consumed in the case of blowing pressurized gas into the water. Namely, according to the present embodiment, energy consumption during vessel operation is effectively conserved by reducing the frictional resistance of hull 60 with a lower level of energy consumption.

[0203] In addition, in the present embodiment, a separation region and cavitation occur due to the level difference formed at the entrance of indentation 420. Consequently, gas and water are aggressively mixed at the interface of the gas and water due to the agitation effects produced by this separation region and cavitation, thereby promoting the release of bubbles 452 from gas-liquid interface.

[0204] Moreover, due to circulating flow Γ that occurs around wing 438 (because the flow rate over the upward facing wing surface is greater than that over the downward facing wing surface), a pressure difference occurs above and below wing 438, and upward lift acts on hull 60 by means of wing 438. Consequently, the bow in particular of hull 60 is raised up due to this lift, the submerged surface area of hull 60 decreases, and the friction resistance of hull 60 is further reduced. In addition, since this circulating flow Γ that occurs around wing 438 acts in the direction that increases the flow rate inside water channel 439, a decrease in the water pressure inside water channel 439 is promoted, and the suction force on bubbles 452 into the water increases.

[0205] Moreover, this circulating flow Γ also occurs around struts 436 and 437 that support wing 438, and these circulating flows Γ cause the formation of an eddy in the water behind them. The eddy formed by circulating flow Γ changes the eddy structure in the turbulent boundary layer in the vicinity of the wall surfaces involved in frictional resistance, thereby dispersing the amount of water movement within the boundary layer and contributing to reduction of the frictional resistance of hull 60.

[0206] Furthermore, since adequate circulating flow Γ is generated even during low-speed operation (for example, at about 10 knots), the above-mentioned friction resistance reduction effects are demonstrated over a wide range of operating velocities.

[0207] In addition, in the present embodiment, the protruding height of wing 438 is adjusted by positioning mechanism 423 according to the operating velocity and other operating conditions, and is controlled so that bubbles are effectively released into the water. This adjustment can be performed easily by a crew member by operating lever 440 arranged on the deck.

[0208] In addition, in the present embodiment, in the case, for example, the vessel has not reached a prescribed vessel velocity or in the case frictional resistance reduction effects produced by the bubbles cannot be expected due to inclement weather, as shown in FIG. 15B, the wing 438 is positioned by positioning mechanism 423 so that the lower surface of wing 438 is roughly at the same height as hull shell plate 62. Consequently, an increase in drag with respect to water flow 450 is inhibited. Namely, by positioning the lower surface of wing 438 so as to be roughly in the same plane as hull shell plate 62, together with the opening of indentation 420 being blocked, since the surface irregularities in hull shell plate 62 are reduced, the increase in drag with respect to water flow 450 is effectively inhibited.

[0209] In this manner, in the present embodiment, the energy consumption during operation can be reduced by effectively releasing bubbles into the water according to operating velocity or inhibiting excessive increases in the drag of hull 60 as a result of controlling the protruding height of wing 438 from hull shell plate 62 according to the vessel operating conditions.

[0210] In addition, since bubble generator 411 employs a simple composition and eliminates the need for an apparatus for pressurizing the gas, it goes without saying that the construction cost of hull 60 can be held to a low level.

[0211] Furthermore, the various shapes and combinations, etc. of each composite member shown in the embodiment described above refer to only a single example, and can be altered in various ways based on design requirements and so forth within a range that does not deviate from the purport of the present invention. For example, although positioning mechanism 423 is operated manually by means of lever 440 based on operating conditions in the above embodiment, the present invention is not limited to this, but rather the protruding height of wing 438 may also be adjusted by driving positioning mechanism 423 automatically based on operating velocity and other data.

[0212] In addition, although the above embodiment indicated an example of applying the present invention to a small vessel, the present invention is not limited to large ships, but can also be applied to other vessels such as tankers, container ships and other oversized and large ships as well as high-speed vessels. The size, number and location of bubble generator 411 should be suitably determined according to the shape of the hull.

[0213] In FIGS. 17A and 17B, recesses are formed in hull shell plate 62 (the vessel bottom here) to serve as bubble generators in addition to depressions 420 in which the above wing body is arranged. Here, a plurality of recesses 70 having a prescribed depth are formed between a plurality of depressions 420 arranged in a row in the direction of width and to the outside of a plurality of depressions 420. As shown in FIG. 17B, these recesses 70 are formed so that their depth from hull shell plate 62 gradually becomes shallower towards the direction of the stern (towards the back in the direction of progress). In addition, inclined surface 71, which protrudes at a prescribed height from hull shell plate 62, is formed in the area in front of (bow side) and adjacent to each recess 70. The angled portion of this inclined surface 71 causes the occurrence of separation and cavitation for the purpose of newly generating bubbles in the water and guiding bubbles released from the above indentation 420 into recess 70 by creating a state of low pressure inside recess 70. In this case, a portion of the bubbles released from indentation 420 fill the inside of recess 70 and are dispersed in the direction of vessel width. As a result, a broad range of hull shell plate 62 is covered by dense bubbles which is able to promote frictional resistance reduction effects.

[0214] Furthermore, the various shapes and combinations, etc. of each composite member shown in the embodiment described above refer to only a single example, and can be altered in various ways based on design requirements and so forth within a range that does not deviate from the purport of the present invention. The shapes of bubble generator 411 and recess 70 are designed using various types of analysis such as flow field analysis by computational fluid dynamics so as to minimize resistance (drag) relative to the flow of water accompanying operation of hull 60 as well as change that flow of water to a desired state.

[0215] Furthermore, since bubbles 22, 52, 142, 342 and 452 that are mixed into the water in the above embodiments 1 through 5 are formed at an internal pressure lower than the hydrostatic pressure corresponding to water depth, when the above bubbles move at a constant water depth (for example, when the bubbles are moving along the vessel bottom), large water pressure acts on the bubbles the farther they move away from the negative pressure region, thereby causing the size of the bubbles to gradually become smaller. According to research conducted thus far by the present applicants, comparatively small bubbles are preferable for reducing the frictional resistance of hull 60. Thus, bubbles generated by negative pressure also act advantageously for reducing frictional resistance with respect to this point as well. 

What is claimed is:
 1. A frictional resistance reducing vessel that reduces frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull, provided with: a negative pressure forming portion arranged protruding from said submerged surface that forms a negative pressure region in the water which is at low pressure relative to a gaseous space; an air outlet for releasing bubbles towards said negative pressure region; a flow path of which one end is open to a gaseous space and the other end is open into the water via said air outlet; and, a drive mechanism that changes at least one of the protruding state of said negative pressure forming portion from said submerged surface, the opening surface area of said air outlet, and the flow path cross-sectional surface area of said flow path.
 2. The frictional resistance reducing vessel according to claim 1 that is also provided with a control apparatus that controls said drive mechanism based on changes in vessel speed.
 3. A method of reducing frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull; wherein, a negative pressure region at a low pressure relative to a gaseous space is formed in the water accompanying vessel operation, and together with a gas being led from the gaseous space to the negative pressure region in the water, the state of said negative pressure region is changed based on changes in vessel speed.
 4. A frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull, provided with: an outer cylindrical portion of which one end is open to the atmosphere and the other end is open in the water; an inner cylindrical portion, which together with being installed in the state of being inserted into the outer cylindrical portion, one end is open to the atmosphere and the other end is open into the water; and, an asymmetrical air blowing portion provided on the other end of said inner cylindrical portion protruding from said submerged surface, the protruding state of which is in the lengthwise direction of the hull.
 5. The frictional resistance reducing vessel according to claim 4 that is also provided with a protrusion adjustment means for adjusting the protruding state of said air blowing portion.
 6. The frictional resistance reducing vessel according to either of claims 4 of 5 wherein, the cross-sectional shapes of said outer cylindrical portion and said inner cylindrical portion are isosceles triangles.
 7. A frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull, provided with: a negative pressure forming portion arranged protruding from said submerged surface so that cavitation occurs in the water behind it due to the relative flow of water with respect to the hull during operation; a discharge outlet provided behind said negative pressure forming portion; and, a flow path of which one end is open to a gaseous space and the other end is open into the water via said discharge outlet.
 8. A frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull, provided with: an indentation formed so as to be recessed from the submerged surface; a negative pressure forming member, which together with being supported to as to rotate freely inside said indentation, forms a negative pressure region in the water at a low pressure relative to a gaseous space by having at least a portion protrude from said submerged surface; a flow path for guiding air from the gaseous space to the negative pressure region in the water, one end of which being open to the gaseous space, and the other end being open into said indentation; and, an angle adjusting mechanism for causing at least a portion of said negative pressure forming member to protrude in a prescribed state from said submerged surface that supports said negative pressure forming member and adjusts the angle of said negative pressure forming member.
 9. A frictional resistance reducing vessel that reduces the frictional resistance of a hull by releasing bubbles onto the submerged surface of a hull, provided with: an indentation provided in said submerged surface; a flow path of which one end is open to the atmosphere and the other end is open to the inside of said indentation; a wing body having a wing that is arranged within said indentation; and, a positioning mechanism that supports said wing body in a prescribed direction while allowing to move freely and positions said wing at a prescribed position; said positioning mechanism positioning said wing so that the inside of said indentation is open relative to the flow of water and results in negative pressure relative to the atmosphere during reduction of frictional resistance.
 10. The frictional resistance reducing vessel according to claim 9 wherein, said positioning mechanism positions said wing so that the lower surface of said wing is at roughly the same height as said submerged surface during times other than when reducing frictional resistance.
 11. The frictional resistance reducing vessel according to either of claims 9 or 10 wherein, said switching mechanism has an operational means that allows the position of said wing to be freely manipulated from the deck. 